A high speed (10 to 25 mm/sec) thermal recording apparatus, which provides high definition images equal to those obtained by electrostatic plotters according to a CAD/CAM system, has been developed for use in place of an electrostatic plotter. This thermal recording apparatus is now sold at about half the price of electrostatic plotters.
Advantages of such a raster system thermal recording apparatus are that the same drawing can be continuously output by using a repeat function, which avoids the need to take diazo copies, and an opaque thermal recording material can be used. A thermal recording material conventionally used in this type of recording apparatus is usually prepared by first calendering natural pulp paper having a Bekk's index of not more than 120 seconds to smooth the surface to a Bekk's index of from 150 to 1,100 seconds, providing a thermal (heat-sensitive) recording layer thereon, followed by drying, and then further calendering the thermal recording layer. In order to improve the ability to preserve originals and print at high speed, the use of opaque synthetic paper having an opacity of from 90 to 95% has been suggested. Such paper is known to be useful as a support for a thermal transfer image-receiving sheet as described in JP-A-63-222891, JP-A-63-290790, JP-A-63-307988 and JP-A-63-315293 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
A thermal recording material, comprising the above-mentioned opaque synthetic paper as a support, has a high Bekk's index (600 to 2500 seconds; measured according to JIS P-8119), excellent high-speed printability, and record preservability. However, such a material is too smooth to allow for satisfactory pencil writing or erasure. There has been a need to improve these properties. There has also been a need to develop a semi-transparent thermal recording material capable of being copied onto diazo-type paper, like the recording material for a CAD electrostatic plotter.
The present inventors previously proposed a semi-transparent thermal recording material capable of being copied in a diazo process, which comprises a synthetic paper support having thereon a thermal color forming layer (see JP-A-3-190787). This support comprises a biaxially stretched resin film base layer, having on both sides thereof a uniaxially stretched thermoplastic resin film containing 10 to 50% by weight calcium carbonate powder as a calcium carbonate powder containing layer. This support has (i) an opacity of not more than 45%, as measured according to JIS P-8138, (ii) a calcium carbonate powder containing layer, on which the thermal color forming layer is formed, that has a Bekk's index of from 100 to 300 seconds and a center-line average roughness (Ra; measured according to JIS B-0601) of not more than 1.5 .mu.m, and (iii) a density of not more than 1.1 g/cm.sup.3, as measured according to JIS P-8118. This previously proposed recording material has already put to practical use.
The above-mentioned thermal recording material is suitable for high-speed printing, capable of being copied in a diazo process, and can be written on with a pencil.
However, this material sometimes causes image defects called white spots. In addition, there is still a need for a semi-transparent thermal recording material with improved dot reproducibility.